Soldering in a reflow furnace is carried out by applying a suitable amount of a solder paste to portions to be soldered of a printed circuit board with a printing apparatus or a dispenser, mounting electronic parts on the portions to which solder was applied, and performing heating in a reflow furnace to melt the solder paste and solder the printed circuit board and the electronic parts to each other.
A reflow furnace is equipped with a preheating zone, a main heating zone, and a cooling zone. When soldering of a printed circuit board is carried out in a reflow furnace, in the preheating zone, solvents in the solder paste are vaporized and thermal shock due to high temperature heating in the main heating zone is reduced. In the main heating zone, solder powder in the solder paste is melted, and it wets and spreads over the portions to be soldered of a printed circuit board. In the cooling zone, the printed circuit board which was heated to a high temperature is rapidly cooled, molten solder is solidified, and electronic parts are protected against the effects of heat.
In general, during soldering of a printed circuit board in a reflow furnace, in the preheating zone, the entire printed circuit board is uniformly heated at a low temperature for a somewhat long time in order to vaporize solvents in the solder paste and to alleviate thermal shock due to rapid heating in the main heating zone, and in the main heating zone, rapid heating is performed in a short length of time to a temperature of at least the melting point of the solder powder in the solder paste. Thus, a reflow furnace abruptly changes from the preheating temperature, which is experienced for a long period, to the main heating temperature, and in the main heating zone, the heating time is shortened so as not to thermally damage electronic parts and printed circuit boards.
Heaters used for reflow furnaces include far infrared heaters using only electrothermal heaters, hot air-blowing heaters which blow hot air from a large number of holes or nozzles, and far infrared hot air-blowing heaters which combine far infrared rays with blowing of hot air.
Far infrared rays can pass to the interior of an object being heated and perform heating, but they cannot reach the lower portions of electronic parts or parts that are in shadows, so it is difficult to perform uniform heating with a heater using only far infrared rays. With a heater using only hot air, hot air can flow around to the bottom of electronic parts or portions that are in shadows to perform heating, but the hot air cannot adequately heat the interior of objects being heated. A heater employing both far infrared rays and hot air utilizes the advantages of far infrared rays and hot air, and it can adequately heat the interior of an object being heated as well as the bottom of electronic parts and portions in shadows, so today it is utilized in many reflow furnaces.
With printed circuit boards which are incorporated into electronic equipment requiring high reliability such as computers or communications equipment, if flux residue remains on the printed circuit boards after soldering in a reflow furnace, the flux sometimes causes a deterioration in the properties of the electronic equipment. This is because activators which are added to flux remain in flux residue. Moisture-absorbing activators easily absorb moisture in air, and they sometimes corrode conductors or reduce the insulation resistance between adjoining conductors. Therefore, it is necessary to clean off flux residue after soldering of printed circuit boards to be incorporated into electronic equipment requiring high reliability.
Organic solvents such as trichloroethylene, Freon, and alcohol are suitable as cleaning agents for flux residue. However, these solvents break down the ozone layer surrounding the earth and produce global warming. Therefore, they are a cause of destruction of the global environment, so their use is being regulated. Accordingly, so-called no-clean solder paste which does not require cleaning of flux residue after soldering is used for soldering of printed circuit boards for high reliability electronic equipment.
No-clean solder pastes employ a minimized amount of activators which causes absorption of moisture, or they employ activators having a weak activating action. At the time of soldering, activators reduce and remove oxide films which cover the portions to be soldered of a printed circuit board or the surface of solder powder and they enable molten solder to adequately wet and spread over portions to be soldered so that soldering can be carried out without defects. In addition, activators completely melt solder powder so as to prevent formation of minute solder balls which can be the cause of short circuits or a reduction in insulation resistance.
A no-clean solder paste can be used with printed circuit boards for high reliability electronic equipment, but if a non-clean solder paste is used in air, the wetting and spreading of the solder paste on portions to be soldered becomes poor, and a large amount of minute solder balls end up forming. This is because oxygen in air has a large influence, and the action of a small amount of activator or a weak activator cannot compensate for this influence. However, if a no-clean solder paste is used in a state without oxygen, i.e., in an inert atmosphere, molten solder adequately wets and spreads over portions to be soldered, and minute solder balls are no longer formed, so good soldering can be carried out. Today, in soldering of printed circuit boards incorporated into electronic equipment requiring high reliability, soldering is often carried out using a non-clean solder paste in a reflow furnace having an inert atmosphere.
In a reflow furnace with an inert atmosphere (referred to below simply as a reflow furnace), it is necessary for the oxygen concentration to be made as low as possible, but in a reflow furnace using a heater which blows hot air, hot air flows into the furnace, and it becomes easy for outside air to enter from the entrance and exit of the reflow furnace, so it is easy for the oxygen concentration to increase and to become unstable. However, a reflow furnace using a heater which blows hot air can more easily form a temperature profile suitable for heating of printed circuit boards than can a reflow furnace using only far infrared rays, so heaters combining far infrared rays and blowing of hot air are much used in reflow furnaces.
In a reflow furnace, heating must be carried out so that the temperature distribution is uniform and temperature differences are small among all locations on a printed circuit board on which electronic parts are mounted. Small electronic parts such as chip parts and large electronic parts such as integrated circuit parts are randomly mounted on a printed circuit board. Small electronic parts have a small heat capacity, so their portions to be soldered rapidly increase in temperature, but large electronic parts have a large heat capacity, so their portions to be soldered experience a slow increase in temperature. The temperature difference between portions to be soldered which rapidly increase in temperature and reach a high temperature early and portions to be soldered which have a slow temperature increase and which do not increase in temperature is referred to as Δt. In a reflow furnace, Δt is preferably as small as possible. This is because if the temperature of the main heating zone of a main heating furnace is set in accordance with the temperature of portions having a fast temperature increase, even if solder paste which is applied to these portions melts, solder paste which is applied to portions having a slow temperature increase does not completely melt, or even if the solder paste melts, the surface activity of the molten solder is low, so it does not completely wet and spread on the portions to be soldered. Conversely, if the temperature is set in accordance with the temperature of portions having a low temperature increase, when solder paste which is applied to these portions is melted, portions having a rapid temperature increase are overheated, and electronic parts and the printed circuit board undergo thermal damage.
There have been many proposals of reflow furnaces such as those in which the size of holes from which air is blown is varied or the position of the holes is varied in order to reduce Δt when using a heater of the type blowing hot air (a heater combining infrared rays and blowing of hot air). In JP H2-137691 A, infrared heaters and nozzles for blowing hot air are installed in a heating portion, the nozzles are installed in a direction perpendicular to the direction of transport, and a large number of small holes provided in the nozzles successively increase in size in the direction of transport. In JP H10-284831 A, the number and area of holes for blowing hot air in a plate for blowing hot air is set so that the amount of hot air which is discharged from the holes is larger on the exit side of the apparatus and is smaller on the discharge side of the apparatus. In JP 2000-22325 A, a mask having a large number of holes formed therein is provided on a discharge port for hot air, and a cover is provided for preventing the passage of hot air to a portion of the mask corresponding to electronic parts which are sensitive to overheating. In JP 2003-33867 A, a large number of through holes are randomly arranged in a plate-shaped material.
A heater which blows hot air in JP H2-137691 A has a structure in which a plurality of pipes are installed in a direction perpendicular to the direction of transport, holes are formed in these pipes, and hot air is discharged from the holes. A heater for blowing hot air in JP H10-284831 A has a structure in which through holes are formed between an outer chamber and an inner chamber, hot air which flows in through the through holes passes through a heating chamber via a suction inlet and enters a hot air supply port, and then hot air is blown out from holes in a plate for blowing hot air installed in the inner chamber. A heater for blowing hot air in JP 2000-22325 A has a structure in which a partition is provided inside a furnace wall, a fan is provided below the partition, a heater is provided on one side of the partition, and a mask having a large number of discharge holes is detachably mounted in the upper portion of the partition. In JP 2003-33867 A, gas is sucked in along both sides of a perforated plate, and hot air is discharged from its center.